1. Field of the Invention
The present invention relates to an image forming apparatus, and, in particular, to an image forming apparatus configured to correct skew of sheets by using a registration roller pair.
2. Description of the Related Art
Conventionally, in an image forming apparatus utilizing electrophotographic method configured to form an image on a sheet, such as a recording sheet, a toner image (visible image) borne by a photosensitive drum or a transfer member is transferred to a sheet, such as plain paper, fed by a sheet feeding device thereby obtaining an image. In such a conventional image forming apparatus, during feeding operation in which the sheet is fed by the sheet feeding device, or during sheet conveyance thereafter, skew of a sheet to the sheet feeding direction may occur. When the skew of a sheet occurs, an image is formed on the sheet in a tilting manner to the sheet direction through transfer and fixing steps.
Therefore, in the conventional image forming apparatus, a skew correction unit is provided on the upstream side of the image forming unit, and skew of the sheet is corrected by this skew feed correction unit. With the skew correction unit, accuracy of an image forming position is improved. Such a skew correction unit causes the leading edge of the conveyed sheet to abut against the nip of a stopped registration roller pair and thereby, the sheet is warped. Consequently, the sheet leading edge is aligned in the direction orthogonal to the sheet feeding direction.
Conventionally, between the sheet feeding device and the registration roller pair, there is arranged a conveyance roller for conveying the sheet fed from a sheet feeding roller of the sheet feeding device to the registration roller pair. However, to achieve a reduction in the size of the apparatus as a whole, there has been proposed an image forming apparatus in which, instead of providing the conveyance roller, the registration roller pair is arranged directly downstream the sheet feeding roller of the sheet feeding device. This technique is discussed, for example, in Japanese Patent Application Laid-Open No. 11-343050.
FIG. 10 is a diagram illustrating an example of a conventional image forming apparatus. Sheets S fed by a sheet feeding roller 51 are fed while being separated one by one by a separation roller 53, which is brought into press contact with the sheet feeding roller 51 by a pressure spring 56. The sheet S fed by the sheet feeding roller 51 is conveyed toward the nip portion of a stopped registration roller pair 57. Then, the leading edge of the sheet S abuts against the nip portion to form a warp in the sheet S. In this way, skew of the sheet leading edge is corrected. After the formation of the warp in the sheet S, the registration roller pair 57 is rotated by an image leading edge synchronization signal. Then, the skew of the entire sheet S is corrected before the sheet S is conveyed to the image forming unit.
In the conventional image forming apparatus, when conveying a sheet of high rigidity, such as an envelope or a postcard, slipping may occur between the sheet feeding roller 51 and the sheet S when rotating the registration roller pair 57 after the formation of the warp in the sheet S. Consequently, defective conveyance of the sheet S may occur.
The above described issue may be caused by the reason that, when the leading edge of the sheet S abuts against the nip portion of the stopped registration roller pair 57 to form the warp in the sheet S, the separation roller 53 is pressed down away from the sheet feeding roller 51 as indicated by the arrow A illustrated in FIG. 10. When the separation roller 53 is pressed down, the press contact force between the sheet S and the sheet feeding roller 51 is weakened. Therefore, the required conveyance force by the sheet feeding roller 51 for conveying the sheet S cannot be obtained. As a result, the leading edge of the sheet S cannot enter the nip portion of the registration roller pair 57, and the conveyance of the sheet S is suspended.